In some implementations, a wireless terminal is configured to communicate directly with another wireless terminal. Such communication is called device-to-device (D2D) communication. The D2D communication includes at least one of direct communication and direct discovery. In some implementations, a plurality of wireless terminals supporting the D2D communication form a D2D communication group autonomously, or under the control of a network, to communicate with another wireless terminal within the D2D communication group.
3GPP Release 12 defines Proximity-based services (ProSe) (see, for example, Non-Patent Literature 1). ProSe includes ProSe discovery and ProSe direct communication. ProSe discovery makes it possible to detect that wireless terminals are in proximity to each other. ProSe discovery includes direct discovery (i.e., ProSe direct discovery) and network-level discovery (i.e., EPC-level ProSe discovery).
ProSe direct discovery is performed through a procedure in which a wireless terminal capable of performing ProSe (i.e., ProSe-enabled User Equipment (UE)) discovers another ProSe-enabled UE by using only the capability of a radio communication technology (e.g., Evolved Universal Terrestrial Radio Access (E-UTRA) technology) possessed by these two UEs. On the other hand, in EPC-level ProSe discovery, a core network (i.e., Evolved Packet Core (EPC)) or a ProSe function (D2D controller 5) determines proximity of two ProSe-enabled UEs and informs these UEs of the detection of proximity. ProSe Direct Discovery may be performed by three or more ProSe-enabled UEs.
ProSe direct communication enables establishment of a communication path(s) between two or more ProSe-enabled UEs existing in a direct communication range after the ProSe discovery procedure is performed. In other words, ProSe direct communication enables a ProSe-enabled UE to directly communicate with another ProSe-enabled UE without traversing a public land mobile network (PLMN) including a base station (eNodeB). ProSe direct communication may be performed by using a radio communication technology that is also used to access a base station (eNodeB) (i.e., E-UTRA technology) or by using a wireless local area network (WLAN) radio technology (i.e., IEEE 802.11 radio technology).
ProSe direct discovery and ProSe direct communication are performed on a direct interface between UEs. This direct interface is referred to as a PC5 interface or a sidelink. That is, ProSe direct discovery and ProSe direct communication are examples of the D2D communication. The D2D communication can also be referred to as sidelink communication or may be referred to as peer-to-peer communication.
In 3GPP Release 12, a ProSe function communicates with a ProSe-enabled UE via a public land mobile network (PLMN) and assists ProSe Discovery and ProSe direct communication. The ProSe Function is a logical function that is used for PLMN-related operations required for ProSe. The functionality provided by the ProSe function includes, for example: (a) communication with third-party applications (ProSe Application Server), (b) authentication of a UE for ProSe Discovery and ProSe direct communication, (c) transmission of configuration information for ProSe Discovery and ProSe direct communication (e.g., EPC-ProSe-User ID) to a UE, and (d) providing of network-level discovery (i.e., EPC-level ProSe discovery). The ProSe function may be implemented in one or more network nodes or entities. In this specification, one or more network nodes or entities that implement the ProSe function are referred to as a “ProSe function entity” or a “ProSe function server”.
3GPP Release 12 further defines a partial coverage scenario where one UE is located outside the network coverage, and another UE is located within the network coverage (see, for example, Sections 4.4.3, 4.5.4, and 5.4.4 in Non Patent Literature 1). In the partial coverage scenario, the UE outside the coverage is referred to as a “remote UE”, and the UE that is in coverage and performs relaying between the remote UE and the network is referred to as a “ProSe UE-to-Network Relay”. The ProSe UE-to-Network Relay relays traffic (downlink and uplink) between the remote UE and the network (E-UTRA network (E-UTRAN) and EPC).
More specifically, the ProSe UE-to-Network Relay attaches to the network as a UE, establishes a PDN connection to communicate with a ProSe function entity or another Packet Data Network (PDN), and communicates with the ProSe function entity to start ProSe Direct Communication. The ProSe UE-to-Network Relay further performs the discovery procedure with the remote UE, communicates with the remote UE on the inter-UE direct interface (e.g., sidelink or PC5 interface), and relays traffic (downlink and uplink) between the remote UE and the network. When the Internet Protocol version 4 (IPv4) is used, the ProSe UE-to-Network Relay operates as a Dynamic Host Configuration Protocol Version 4 (DHCPv4) Server and Network Address Translation (NAT). When the IPv6 is used, the ProSe UE-to-Network Relay operates as a stateless DHCPv6 Relay Agent.
In this specification, a wireless terminal having D2D communication capability and relay capability such as the ProSe UE-to-Network Relay is referred to as a “mobile relay (MR)”, a “mobile relay (MR) mobile terminal (MT))”, or a “mobile relay UE (MR UE)”. Further, a wireless terminal receiving a relay service by the mobile relay is referred to as a “remote mobile terminal (remote MT)” or a “remote UE”.